The mechanism of the synthesis of doubly labeled acetate from CO2 by heterotrophic bacteria is being studied to determine the role of folates and corrinoids and of transcarboxylation in the reaction. Purification of the corrinoid enzymes involved in the synthesis of acetate and of the alpha keto acid methyl corrinoid transcarboxylase will be undertaken from C. thermoaceticum. The synthesis of doubly labeled acetate from CO2 by C. formicoaceticum and C. acidiurici will be investigated to determine if they utilize the same mechanism as C. thermoaceticum.